In recent years, regarding a brushless motor for an axial fan, the needs for high output and high revolution are increased. Accordingly, a problem occurs in that even when power feed from a power supply is interrupted (when cutoff of the power supply, disconnection of a power supply line and the like is caused), it takes for a propeller of the axial fan to stop, so that an accident may occur. In order to handle the such problems, it is required in the brushless motor that a rotation speed of the motor is rapidly lowered to stop the rotation of the motor.
JP-A-2007-259617 discloses an invention of short-circuit braking (short brake) that shortly stops rotation of a motor when interruption of power feed from a power supply.
According to the background short-circuit braking disclosed in JP-A-2007-259617, when the power feed from the power supply is interrupted, all switching elements of an upper arm-side of a driving circuit (inverter circuit) are turned off and all switching elements of a lower arm-side are turned on. Accordingly, an armature coil of the motor is shorted and operates as an electromagnetic brake, so that the rotation of the motor can be stopped in a short time.